Discharge lamps, and particularly discharge lamps which have a metal halide fill, usually have a discharge vessel of quartz glass. Such lamps are particularly suitable for association with optical systems, such as a light beam directing elements, search lights, headlights, or flood lights. Typical power ratings are between about 400 to 4000 W. Lower power lamp types, that is, lamps having a power rating of, for example, 150 W, can also be used for display window illumination or for general service illumination, and particularly where an intense light source of high efficiency is needed, which is not subject to frequent high ON/OFF cycling rates.
The referenced U.S. Pat. Nos. 4,396,857 and 4,806,816 show and describe double-ended, that is, double-sided miniature high-pressure lamps of lower power, that is, lamps having a power rating of about 35 W, and which have an outer bulb or envelope. These lamps have two pinch seals. The discharge vessel volume is less than 1 cm.sup.3. It is desirable to prevent accumulation of excess metal halides at cold spots, in the parts of the vessel behind the electrodes and to ensure precise positioning of the electrodes within the discharge bulbs. To provide for precise positioning and as a means against cold spots, the lamps have pinch seals which join an essentially cylindrical transition region towards the discharge vessel itself. The discharge vessel, usually, is of somewhat bulbous or barrel shape. This transition region is constricted in the plane of the pinch seal, and expanded or extended with reference to the narrow sides of the pinch seal. The transition region, thus, has an increased wall thickness and an accumulation of glass mass with respect to the wall thickness of the discharge vessel as such. The result will be an undesired relatively good heat conduction and heat transmission from the vessel to the pinch seals and, further, a relatively good heat radiation due to the large radiating surface of the cylindrical transition region. The heat damming effect, to be obtained, thus is not entirely satisfactory. Manufacturing such lamps is relatively complicated since the transition region is made in two manufacturing steps. The pinch sealing step itself is done with two pinch jaws.
U.S. Ser. No. 07/500,760 describes a double-sided, that is double-ended, double pinched metal halide discharge lamp intended to have high power, in the order of 1000 to 2000 W, for example, and suitable for operation without an external surrounding bulb or transparent housing. Lamps of that type are particularly critical with respect to heat balance and the heat damming effect is especially important since, otherwise, the halide vapor pressure will not reach its desired value and hence the color temperature of the lamp will not be an optimum, as desired. In part, a heat damming coating is used. This heat damming or heat retention coating, however, increases scattering or spreading of the color of the light emitted by the lamp, and also causes some shadowing by decreasing light transmission. It has been found, in operation, that these lamps, which are intended to be installed without an external vessel, may break at the transition region to the central discharge vessel. The end regions are retained in bases and rough handling of the bases, or shifting in their sockets, may cause the lamps to break.